The present application relates to a thin-film transistor having an organic semiconductor layer, a method of manufacturing the thin-film transistor, and an electronic device using the thin-film transistor.
In recent years, an active matrix drive system has been introduced into many electronic devices represented by a display device, and a thin-film transistor (TFT) is used as an element for the switching (pixel selection).
FIG. 9 illustrates a cross-sectional configuration of a channel etch type of TFT in the past. In this TFT, a semiconductor layer 103 is formed on a gate electrode 101 and a gate insulating layer 102, and a source electrode 104 and a drain electrode 105 are connected to the semiconductor layer 103. The source electrode 104 and the drain electrode 105 are spaced apart from each other and respectively disposed to be overlaid on an upper part of the semiconductor layer 103.
Regions R1 to R3 illustrated in FIG. 9 represent regions determined by a positional relationship between the semiconductor layer 103 and the source electrode 104 as well as the drain electrode 105. The region R1 is where the semiconductor layer 103 overlaps the source electrode 104, and the region R2 is where the semiconductor layer 103 overlaps the drain electrode 105. Further, the region R3 is located between the regions R1 and R2, and where the semiconductor layer 103 overlaps neither the source electrode 104 nor the drain electrode 105. What is meant by these regions R1 to R3 remains the same hereinafter.
The semiconductor layer 103 which is a channel layer has a layered structure in which an upper semiconductor layer 103B is formed on a lower semiconductor layer 103A. The lower semiconductor layer 103A is formed of amorphous silicon, and extends from the region R1 via the region R3 to the region R2. The upper semiconductor layer 103B is formed of amorphous silicon doped n-type and is disposed on each of the regions R1 and R2 so that the respective upper semiconductor layers 103B are spaced apart from each other.
When such a semiconductor layer 103 is formed, first, the lower semiconductor layer 103A and the upper semiconductor layer 103B are formed to extend from the region R1 to the region R2 and then, the source electrode 104 and the drain electrode 105 are formed. Subsequently, the upper semiconductor layer 103B is selectively etched by using the source electrode 104 and the drain electrode 105 as a mask. As a result, a part of the upper semiconductor layer 103B is removed in the region R3 and thus, the upper semiconductor layer 103B remains only in each of the regions R1 and R2.
In this channel etch type of TFT, electrical resistance of the n-type-doped upper semiconductor layer 103B becomes lower than electrical resistance of the lower semiconductor layer 103A not doped n-type. As a result, contact resistance between the source electrode 104 as well as the drain electrode 105 and the upper semiconductor layer 103B decreases and thus, an electric charge becomes easy to go in and out between the source electrode 104 as well as the drain electrode 105 and the semiconductor layer 103.
Incidentally, an organic TFT using the organic semiconductor layer as a channel layer has been receiving attention recently. In the organic TFT, the channel layer can be formed by coating, which makes it possible to reduce the cost. In addition, the channel layer can be formed at a temperature lower than an evaporation method and the like and thus, the organic TFT can be implemented on a low heat-resistant flexible plastic film or the like.
FIG. 10 illustrates a cross-sectional configuration of the organic TFT in the past. This organic TFT has a structure similar to the TFT illustrated in FIG. 9, except that has an organic semiconductor layer 203 in replace of the semiconductor layer 103. Specifically, the organic semiconductor layer 203 is formed on a gate electrode 201 and a gate insulating layer 202, and a source electrode 204 and a drain electrode 205 are connected to the organic semiconductor layer 203. This organic semiconductor layer 203 has a single-layered structure, and extends from the region R1 to the region R2.
As a structure of this organic TFT, like TFTs in the past, various kinds of structure such as a top contact type, a bottom contact type, a top gate type and a bottom gate type are considered. Above all, the top contact type in which the source electrode and the drain electrode are disposed to be overlaid on an upper part of the organic semiconductor layer is common (for example, see International Publication WO2007/055119)